The present invention relates in general to a two-phase quality/flow meter which can measure the ratio of liquid to gas in, and the flow velocity of, a two-phase flow stream by using capacitance measurements.
Many prior art measurement devices monitor capacitance changes to measure a physical phenomenon related thereto. For example, because different materials typically have different dielectric constants, a capacitance measurement performed on a mixture of two materials can be employed to determine the ratio of one material to the other. Similarly, the dielectric constant of a single material typically changes when the material changes phase between a liquid and a gas or a liquid and a solid. Thus, the ratio of one phase to another phase in a two-phase flow stream can also be determined using capacitance measurements. This measurement technique is useful in "quality" meters which measure the volume ratio of a gas or liquid component in gas-liquid mixture to the total volume of the mixture, where quality is defined as this ratio. A specific application of a quality meter is in the monitoring of a cryogenic flow of liquid nitrogen, hydrogen or oxygen as is typically used for fueling rocket engines. Cryogenic cooling is required to maintain these elements in their liquid state due to their very low boiling temperatures. Even the slightest malfunction in the cryogenic cooling system can result in the generation of gas bubbles in the flow stream, and these can be very detrimental to the proper operation of the fuel supply system. Accordingly, a means must be provided which can rapidly detect when the proportion of liquid volume to the total volume in the flow stream drops below an unacceptable level, and quality meters provide an accurate means for monitoring such a condition.
Capacitance measurements can also be employed to measure the flow velocity of a material flow stream by making capacitance measurements with two capacitance sensors disposed at spaced locations in the flow stream, and cross-correlating the obtained measurements. The cross-correlation determines when like portions of the flow stream pass each capacitance sensor, and the flow velocity can be determined by dividing the distance between the two sensors by the time interval required for like portions of the flow to pass from one sensor to the other.
Prior art capacitance based measurement devices of these type suffer from a number of drawbacks. Typically, these devices employ a capacitance probe which is positioned in the flow or material to be tested. The probe includes two spaced conductive plates which form the capacitor, and the material to be tested forms the dielectric between the plates. Thus, if the material's dielectric constant changes, such as may occur as a result of a partial phase change, the value of the capacitor also changes.
Numerous techniques have been employed to measure the capacitance of the capacitor in these devices. In a first known technique, the capacitor is connected as part of an RC oscillator circuit whose frequency changes in proportion to changes in the capacitor's dielectric constant. Although this frequency monitoring technique has the benefit of simple electronics, it has the detriment of requiring that a division be performed, which is difficult with analog circuitry. Additionally, this type of device typically employs a high frequency oscillator which is very sensitive to electrical noise induced errors. Further, temperature and pressure changes affect the device capacitance, and these parameters are thus also a source of error.
Another prior art capacitance measurement technique employs a high frequency oscillator to rapidly charge and discharge the capacitor, and the current required to do this is measured. If the capacitor changes in value, a differing amount of current will be required to charge and discharge it. This technique allows high speed monitoring of the capacitance value, but has the detriments of requiring a high speed, accurate and stable oscillator, voltage-to-current monitoring at high speed, and an accurate, high resolution analog to digital converter. Again, the high frequency oscillator is very sensitive to electrical induced errors, and its stability is affected by various factors, such as temperature and pressure changes. Also, the high resolution analog to digital converter increases both the device complexity and its cost.